1. Field of the Invention
This invention relates to a process for polymerizing an .alpha.-olefin in liquid phase using a novel catalyst.
2. Description of the Prior Art
It is known that solution polymerization is suitable for the production of polyethylene and its advantages are as follows;
(1) The polymerization of ethylene is an exothermic reaction and removal of heat is a big problem from the viewpoint of a process. Since the efficiency of removing heat increases with greater differences between the inner temperature of a reactor and that of a cooling jacket, solution polymerization in which a high polymerization temperature is employed is advantageous from this point.
(2) The degree of polymerization of ethylene, i.e. the molecular weight of polyethylene can be controlled comparatively accurately by varying the polymerization temperature and furthermore, the control of the molecular weight of polyether can be done by using a small amount of hydrogen.
(3) Since the molecular weight of polyethylene is correlated with the viscosity of the reaction solution, it can be estimated by measurement of the viscosity of the reaction solution in the reactor and the control of the molecular weight of polyethylene can be quickly done accordingly.
(4) Polyethylene is used or sold usually in the form of pellets. The polyethylene obtained by suspension polymerization and gas phase polymerization is powdery and it is necessary to melt-mold the powdery polyethylene into pellets by an extruder. On the other hand, according to solution polymerization it is possible to remove the polymerization solvent by evaporation with the use of the heat of polymerization and to introduce the polyethylene in its melt form into an extruder. As a result, excess step and heat for melting the polyethylene can be omitted. In order to make the most of this advantage, it is preferred that the polymerization temperature is high.
(5) In producing low density polyethylene by copolymerization of ethylene and an .alpha.-olefin by a slurry method, the formed polymer becomes easily soluble in the polymerization solvent and the polymerization reaction mixture in the reactor becomes gruel, resulting in a difficulty in continuing polymerization. Accordingly, it is difficult to produce polyethylene having a density less than about 0.945. On the other hand, according to solution polymerization, polymerization is conducted at high temperatures in the form of a complete solution without the above described problem and thus it is possible to produce polyethylene having a wide range of density from about 0.975 to about 0.910.
The disadvantage of solution polymerization is an increase in the solution viscosity due to the increase in the solution concentration or the molecular weight of polyethylene, which renders commercial scale production of polyethylene difficult. In order to avoid this disadvantage, it is necessary that the polymerization temperature is raised and simultaneously the solution viscosity is reduced. However, with increased polymerization temperatures the catalyst efficiency is decreased and a large amount of catalyst residue remains in the formed polyethylene and causes discoloration of the polyethylene and deterioration of molded articles obtained therefrom. Further, removal of catalyst residue is difficult. Thus, there are required catalysts having a high catalyst efficiency at high temperatures which enable complete omission of the catalyst removal step due to a small amount of catalyst residue present in the formed polyethylene.
There are known many Ziegler type catalysts having a high catalyst efficiency for suspension polymerization (see, e.g. U.S. Pat. Nos. 4,115,319, 4,159,965 and 4,163,831). However, the catalyst efficiency of these catalysts is, in general, decreased with increased polymerization temperatures, and especially at temperatures higher than about 150.degree. C., the decrease in the catalyst efficiency is remarkable. Thus the performances of such catalysts are not enough to omit the removal step of catalyst residue when employed in solution polymerization.
There are also known catalysts for solution polymerization of an olefin which comprise an organomagnesium complex, an aluminum halide, hydrogen chloride, a halogenated secondary or tertiary alkyl of halogenated silicon compound and a titanium compound (see, e.g., U.S. Pat. Nos. 4,159,965 and 4,172,050 and U.K. Pat. Nos. 1,251,177 and 1,235,062). These catalysts have a higher catalyst efficiency than the conventional catalysts but their catalyst efficiency at high temperatures is still insufficient.
As a result of the study on the catalyst system for solution polymerization it has been discovered that by using, as a catalyst, a component having been obtained by reacting a specific organomagnesium compound with a halide and contacting the obtained product with a titanium compound and/or a vanadium compound, in combination with an organometal component, there can be obtained catalysts having a very high catalyst efficiency without any decrease at least at 150.degree. C., especially at least at 180.degree. C., and an excellent storage stability suitable for the polymerization of an olefin.